Ethernet Fundamentals - Part 1

Ethernet is by far the most popular and widely used LAN protocol today. It was invented by Robert Metcalfe during the 70's and was developed hand in hand by DIX (DEC, Intel, Xerox). Robert Metcalfe would later on found 3Com Corporation, one of Cisco's competitors in the world of networking. Coincidentally, 3Com pertains to "Computers, Communication and Comapatibility" just a slight trivia there. This is so because Mr. Metcalfe is an advocate for compatibility of products around the industry.

The very first ethernet design as presented by Mr. Metcalfe.

What makes Ethernet a very successfull protocol is its simplicity and robustness in adapting to LAN evolution specially in terms of speed. Todays LANs are now able to transmit up to 10 Gbps. The following shows a table of IEEE 802.3 technologies.

Ethernet essentially is a broadcast protocol. Every host in the network has the ability to transmit anytime it wants. It's called Non-deterministic or others call it as opportunistic LAN. It's a first come first serve basis type of communication unlike in Token Ring networks or FDDI (Fiber Distributed Data Interface) wherein a token is needed by hosts in order to transmit one at a time. In Token Rings or FDDI, the hosts cannot transmit without the token. Transmission is in order, that is, hosts transmit taking each others turn. This is called deterministic LAN.

We can probably say that the Ethernet network is chaotic in nature. CSMA/CD takes the role of policing the Ethernet in order for the different nodes to transmit properly. However, the chaotic nature of Ethernet is only true to half-duplex transmission. The following illustrates the three methods of transmission.

Half-duplex Ethernet nowadays is a gonner. It's almost extinct because of the advent of better, cheaper and more intelligent switches. Plus, it is also driven by bandwidth-hungry applications all over the workplace increasing the demands on faster and more efficient transmissions.

Half-duplex transmission practically generates collisions on LAN. Susceptible to this are hosts that are connected in bus topologies or star topologies using hubs or muli-port repeaters. And when there are collisions, CSMA/CD goes to work. Collisions are detected on the LAN by voltage spikes. Full-duplex transmission essentially frees the LAN of collisions and hence no place for CSMA/CD to work on. The hosts on these LANs are connected on switches.

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The MAC Address

The MAC (Media Access Control) address or hardware address is burned in (hence is also called burned-in address) on every Ethernet Network Interface Card or NIC and is unique on every card. It is used by the Ethernet protocol for addressing in the Layer 2 OSI model. It is divided into two parts. The Organizational Unique Identifier (OUI), which is the first 6 HEX digits assigned by IEEE and the second 6 is assigned by the vendors themeselves to uniquely represent the NIC within the OUI. Every ethernet device will have this in its system.

Finding the MAC address or Physical address of your PC is fairly easy. Just go to the command prompt or DOS prompt and type "ipconfig /all" without the quotes.

You can also learn the MAC address of those PCs or devices which your PC recently had communication with by looking at the arp cache table using the arp -a or arp -g command in the command prompt.

Though it is not recommended, there are some ways to change the MAC address of your PC through software methods. It is important to take note that changing the MAC address can adversely affect the stability of your network. One long method is via the windows registry. You can refer to this site for the proper procedure. Or you can change it through the network card properties. But if you are not comfortable changing it, you can always try MAC spoofing such as SMAC. The SMAC software will try to hide your real MAC address from your network.

Why does anyone bother changing the MAC address anyway? Here are some of the reasons why:

1) for personal privacy since some companies do track users via their MAC addresses. However it is also a technique being used by hackers to hide themselves or to spoof other machines.
2) for performing security vulnerability and penetration testing.
3) to build high-availability netowrk solutions such as devices that run multiport NICs

If you are curious enough to find the OUIs assigned to some companies such as Cisco you can try this link: IEEE OUI and Company_ID Assignments. Just type the company name in the search box.

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